This invention relates to information retrieval systems.
Information retrieval systems are known in which records are retrieved from a file by searching the file with a search criterion which specifies certain features of the records which it is required to retrieve. The system examines each record in the file (either simultaneously or sequentially), comparing each record with the search criterion and producing a "hit" indication whenever it finds a record which satisfies the search criterion.
It has been proposed to store the hit indications in a register having one bit location for each record in the file, for example to enable the result of one search to be combined with the result of subsequent searches. However, such previous proposals suffer from the disadvantage that, when the number of records in the file is very large (1 million for example) the register must also be very large. One object of the invention is therefore to provide an information retrieval system in which such hit indications are stored in a more economical manner.
In other words, the invention seeks to provide a system in which the total number of bit locations required to store the hit indications is substantially smaller than the number of records, thus leading to a considerable economy in bit locations compared with previous proposals which require one bit per record.